The liquid crystal display is a device representing images by liquid crystals inserted between two sheets of thin glass substrates. In said device, when voltages are applied through electrodes connected to liquid crystals, the molecular alignment manner of liquid crystals changes, whereby transmission ratio of lights passing through the liquid crystals is changed, so that pictures or colors may be represented. Such a liquid crystal display has advantages that it uses very little power and can be flatly and thinly made. Therefore, it is a display device being in the limelight of various fields at present.
For preparing the liquid crystal display, liquid crystal cells comprising liquid crystals and glass substrates having transparent electrode, and polarizers are basically required and suitable adhesives or pressure-sensitive adhesives are also required for binding them.
The polarizer comprises an iodine compound or a dichroic polarizing material aligned in a certain direction, and has multi-layer structure comprising TAC (triacetyl cellulose) protective films for protecting polarizing elements, and the like. In addition, the polarizer may additionally comprise a phase difference film, a compensation film for wide view angle or a brightness enhancement film, and the like. Each film constituting these multi-layer polarizers is made of materials having different molecular structures and compositions, and so has different physical properties. Thus, under high temperature and/or high humidity conditions, it has inferior dimensional stability, since shrinkage or expansion behavior of the materials having a unidirectional molecular alignment is different. Therefore, if the polarizer is fixed by a pressure-sensitive adhesive, stress is concentrated on the TAC layer by shrinkage or expansion under high temperature and/or high humidity conditions, thereby birefringence and light leakage phenomenon occur.
As a representative method for solving said problems, there is a method for giving stress relief property thereto by designing the pressure-sensitive adhesive to have high creep against external stress and to be easily modified. Specifically, it is a method for mixing high molecular weight polymer containing a functional group which may react with a cross-linking agent, with low molecular weight material including less or no cross-linkable functional group (KR Laid-open Patent Publication No. 1998-79266, and JP Laid-open Patent Publication Nos. 2002-47468 and 2003-49141).
However, the pressure-sensitive adhesive composition disclosed in said techniques has very poor tailoring property. Thus, on preparing polarizers, crooking or pressing phenomenon is arisen in the pressure-sensitive adhesive. So, there is a problem that yield is highly lowered.
As other technique for preventing light leakage phenomenon, there is a method for designing the pressure-sensitive adhesive to be very hard. When the pressure-sensitive adhesive has hard property, shrinking or expansion of the polarizer under high temperature and/or high humidity conditions is suppressed, whereby the resulting stress is minimized and focused on the outermost, so that relatively good optical properties may be realized.
However, in order to design the pressure-sensitive adhesive to be hard, bulk modulus of the adhesive should be much increased, and thereby endurance reliability is deteriorated, since adhesive property becomes lowered according to the increasing of the modulus.
Also, there are proposed methods, in which a photoinitiator and a multi-functional acrylate are added to a conventional single crosslinking structure, so as to improve bulk modulus, since the conventional single crosslinking structure cannot realize enough bulk modulus to maintain good light leakage-resistance property and endurance reliability (JP Laid-open Patent Publication Nos. 2007-197659 and 2007-212995).
In case of the pressure-sensitive adhesive compositions disclosed in said publications, they all employ an activating energy ray-curing system. That is, all pressure-sensitive adhesive compositions of the publication comprise photoinitiator as essential component, and the photoinitiator transforms a multi-functional acrylate into polymer, so as to form a secondary crosslinking structure by UV irradiation.
However, in case of UV curing system as above, it is impossible to get uniform pressure-sensitive adhesive layer, since UV absorbing degrees is changed according to thickness of pressure-sensitive adhesive. Therefore, in case of a polarizer to which pressure-sensitive adhesive disclosed in the publication is adhered, lots of light leakage phenomena are generated at outmost thereof especially when it is adhered to a large size display device, and kept for long time under high temperature and high humidity conditions.